Ultrafine particles constitute the key building blocks for diverse advanced structural and functional materials, such as high-performance ceramics and alloys. These advanced materials have tremendous impact in many areas, including catalysts, separations, electronics, energy production processes, and environmental applications. Of particular importance, nanophase ceramic or metallic materials that contain nanosized (&lt;100 nm) particles/grains show dramatically improved performance (mechanical, electrical, optical, magnetic, and/or catalytic). The characteristics of ultrafine particles, i.e., size, morphology, monodispersity, purity, and homogeneity of composition directly determine the properties of the materials that are made from them. Thus, the future application of advanced materials depends on the capability to produce particles with outstanding characteristics.
Currently, there is a strong need for more efficient methods of production of high-quality inorganic particles. Ideally, an instantly reactive, continuous process that generates homogeneous ultrafine particles with controllable characteristics is desired. The primary technologies for synthesis of ultrafine particles are liquid-phase chemical and sol-gel processing, and gas-phase condensation. Most of the production processes for both approaches are conducted in batch mode. Gas-phase reactions typically require extreme conditions such as high vacuum and high temperature and give very slow particle production rate. A few continuous, liquid-phase processes have been developed for production of microspheres from alkoxide; however, these involve relatively slow kinetics during hydrolysis and condensation, typically 30 minutes or more reaction time. In contrast, real metal alkoxides are so reactive that agglomerated solids, rather than dispersed particles, are formed under conditions with rapid reaction kinetics. Thus, controlled hydrolysis/condensation of alkoxides in a batch reactor is the usual approach for the production of monodispersed metal oxide precursor powders.
Tubular-type reactors have been designed for the continuous synthesis of ultrafine ceramic particles such as titania and ferric oxide via hydrolysis and condensation of metal alkoxides. In addition, liquid spraying techniques including electrostatic spraying/atomization and ultrasonic spraying of liquids into gas have been used in ceramic particle production.